Relocate SKILL.md rationale into docs/RATIONALE.md

[rmichaelthomas]

Summary

Relocates rationale-only prose out of the SKILL.md body into a new docs/RATIONALE.md, leaving every operational rule in place. SKILL.md loads on every session (Claude Code + Codex), so the win is a leaner per-session read. This is a discretionary optimization, not a bloat fix — at 295 lines the body was already under the ~500-line progressive-disclosure ideal.

Passages moved (verbatim → docs/RATIONALE.md)

Passage	From SKILL.md section	Anchor
v1-regression history (Sonnet 4.6 benchmark story)	two-channel protocol	#two-channel-history
Helper "universal floor" design philosophy	contract lifecycle helper	#universal-floor
"What this skill is not" positioning bullets	(eponymous section)	#what-this-skill-is-not

The load-bearing rule "Do not narrate the contract in prose" stays in SKILL.md — only the story that justified it moved. Each excision left exactly one pointer line.

Size delta (SKILL.md)

Metric	Pre (main)	Post	Delta	%
Lines	295	289	−6	2.0%
Chars	23,642	22,441	−1,201	5.1%
Est. tokens (chars/4)	5,910	5,610	−300	5.1%

Pre/post figures saved under benchmarks/.

Invariants satisfied

1. Every operational rule from SHA cfab952 present verbatim — KEEP-region diff is empty.
2. Every moved passage present verbatim in docs/RATIONALE.md.
3. Each excision has exactly one pointer; all three anchors resolve (no dead links).
4. Only SKILL.md (modified), docs/RATIONALE.md, and the two benchmarks/ files changed.
5. Over-cut gate (§5): line reduction (6) < total MOVE lines (~11+) ⇒ no operational content lost.

Verification

• python3 -m pytest tests/ → 35 passed
• sh tests/test_contract_session_init.sh → 8/8 ok
• §8 behavior check (3 scratch sessions on the trimmed skill):
  • Start contract → opens contract, emits Channel-2 limn block, declares lists before add ✓ (minor prose narration leak observed; rule itself unchanged — reviewed and accepted at the manual gate)
  • Fake "99 reserved words" claim → refused to fabricate a cite, disclosed unverified in prose, cited only the true figure (58) ✓
  • Session end → final contract with baseline declarations, narrated save + consent gate, closed contract ✓

🤖 Generated with Claude Code